1. Field of the Invention
The present invention relates to a measuring method and apparatus capable of obtaining information concerning absorption inside a scattering medium like living tissues and the like, an imaging method and apparatus, a fluoroscopic method and apparatus, a tomographic image taking method and apparatus, or a mammographic method and apparatus. More particularly, the present invention concerns a method and apparatus for measuring a temporal change or a spatial distribution of concentration of a specific absorptive constituent in a specific portion inside a scattering medium and further concerns an absorption information measuring method and apparatus of scattering medium capable of non-invasively measuring a concentration distribution of a specific absorptive constituent inside a scattering medium having surfaces of various shapes.
2. Related Background Art
There are four types of main methods for measuring or imaging a living tissue of a scattering medium by use of light, which are roughly classified into (a) one using translationally propagating light (ballistic photon), (b) one utilizing a coherent component by heterodyne detection thereof, (c) one utilizing scattered light, and (d) one utilizing a time-resolved gate.
(a) The method utilizing the translationally propagating light (ballistic photon) is arranged to make pulsed light incident to a measured object and to extract a translationally transmitted component in output light by an ultra-high-speed gate. On this occasion, a flight time of the translationally transmitted light is shorter than those of scattered light and is shortest among the output light. Therefore, the translationally transmitted light can be detected by cutting out a light part first emerging from the measured object by the ultra-high-speed gate. This method has such an advantage that a high spatial resolution as in X-ray CT can be expected theoretically. However, almost all light is scattered multiply inside the measured object being the scattering medium, and therefore, the translationally transmitted component exists little or, even if present, in a very small amount. As a result, the utilization factor of the light upon measurement becomes very poor and, considering the maximum light incidence intensity (approximately 2 mW/mm.sup.2) that can be injected into a living body, an extremely long time becomes necessary for imaging or measurement or the like, which is not practical. With a large object like the head or the arm the signal light is very weak and measurement is impossible in practice.
(b) The method utilizing the coherent component is arranged to split coherent high-frequency modulated light into two components and make one incident to the measured object. Then the output light is made to interfere with the other high-frequency modulated light component to heterodyne-detect the coherent component included in the output light, and the coherent component is used for measurement.
Specifically, since in this measuring system the flight time (or optical pathlength) and the flight direction of scattered light are disordered depending upon the degree of scattering, multiply scattered light does not interfere, and thus nearly translationally transmitted light is detected. Also in this case, therefore, the high spatial resolution is expected theoretically as in the previous example, but the utilization factor of light is very poor, so that it is very difficult to practically apply it to measurement or imaging or the like of living body. It is also practically impossible to measure a large object like the head or the arm.
(c) The method for detecting the scattered light is arranged to make continuous wave (CW) light, pulsed light, or modulated light incident to the measured object and to detect the light (output light) having propagated as scattered in the measured object. On this occasion, where the pulsed light of a sufficiently shorter pulse width than the time response characteristic of the measured object is made incident thereto, it may be considered as incidence of impulse light and the output light corresponding thereto is expressed by an impulse response. The measurement of this type is called as time-resolved spectroscopy (TRS), wherein internal information is calculated by measuring a time-resolved waveform of the impulse response. The method for making the continuous wave (CW) light incident is called as CW spectroscopy and the measurement for making the modulated light incident as phase modulated spectroscopy (PMS). The former measures a time integration value of the impulse response and the latter measures a Fourier transform of the impulse response, i.e., a system function thereof. From the above discussion, it is understood that the time-resolved spectroscopy (TRS) obtains the signal containing more information than the other two methods. The phase modulated spectroscopy (PMS) for sweeping the modulation frequency and the time-resolved spectroscopy (TRS) of impulse response include nearly equivalent information. Since the continuous wave (CW) spectroscopy and the phase modulated spectroscopy (PMS), though greatly different in their signal processing methods, obtain signals including substantially equivalent information, almost equal information can be measured thereby. Since all of these methods utilize almost all output light, their utilization factors of light are increased remarkably as compared with the foregoing two examples of (a) and (b). However, the scattered light spreads in the almost all area or in a considerably wide region inside the measured object, so that the above methods cannot be applied to quantitative measurements in a narrow, specific portion. In imaging or optical CT, the spatial resolution is very poor and practical application is difficult.
In contrast with the above, (d) the method utilizing the time-resolved gate uses a part of the impulse response waveform. For example, photons included in the initial portion of the time-resolved output signal (impulse response waveform) are those little scattered (photons little tarrying on the way), which are photons having propagated near the straight optical path connecting a light incidence position (light incidence point) with a photodetection position (light receiving point). Accordingly, by utilizing the signal from the time-resolved gate, the photons having propagated near the above straight optical path and then detected can be extracted, thus improving the spacial resolution of imaging or the like. The extremity of this is the translationally propagating light (ballistic photon) described previously. The time-resolved gate methods having been developed or proposed heretofore do not permit the quantitative measurements of an absorptive constituent inside the scattering medium. The reason is that no method is developed for analytically describing the signal obtained by the time-resolved gate method. Namely, there are no known methods for analyzing the signal obtained by the time-resolved gate method based on the photon diffusion theory and for utilizing it for quantification of absorptive constituent. Even if a method of analysis based on the photon diffusion theory is developed, there will remain the essential problem resulting from the basis placed on the photon diffusion theory; i.e., there will remain a problem that it is impossible to measure scattering media having various contours and those to which diffusion approximation cannot be applied. From the above, the time-resolved gate method can improve the spatial resolution, but has the serious problem that the internal information of an absorptive constituent or the like cannot be quantified.
There are many references as to the time-resolved gate method as described above and typical examples thereof are listed below. It is, however, noted that none of them presents discussions or suggestions on the quantification of absorptive constituent as disclosed in the present invention.
1) A. J. Joblin, "Method of calculating the image resolution of a near-infrared time-of-flight tissue-imaging system," Appl. Opt. Vol. 35, pp. 752-757 (1996).
2) J. C. Hebden, D. J. Hall, M. Firbank and D. T. Delpy, "Time-resolved optical imaging of a solid tissue-equivalent phantom," Appl. Opt. Vol. 34, pp. 8038-8047 (1995).
3) G. Mitic, J. Kolzer, J. Otto, E. Plies, G. Solkner and W. Zinth, "Time-gated transillumination of biological tissues and tissue-like phantoms," Appl. Opt. Vol. 33, pp. 6699-6710 (1994).
4) J. C. Hebden and D. T. Delpy, "Enhanced time-resolved imaging with a diffusion model of photon transport," Opt. Lett. Vol. 19, pp. 311-313 (1994).
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